A good number of embodiments of generic type devices for insufflating gas are known. With regard to this, reference is made to DE-A-30 00 218, EP-B-O 169 972, DE-C-36 11 018, U.S. Pat. No. 4,874,362 and DE-A-43 19 859. Moreover, reference is explicitly made to these printed publications for the explanation of all details not made more apparent herein.
A typical device for insufflating gas 10 into a corporeal cavity 30, as shown in FIG. 3, possesses a connection for a gas source 32, such as a pressure cylinder, and the capability of setting the gas pressure and the gas flow. For this purpose, the known devices are provided with, for example, a pressure and the gas flow sensors. Also, the known devices are provided with, a pressure/gas flow regulator 34 which reduces the pressure of the gas source to a insufflation pressure (desired gas pressure), which can be set, of typically between 0 and 50 mm Hg, and a flow regulator which sets the gas flow to a value (desired gas flow).
In the generic devices described in the aforementioned printed publications, the gas pressure or the gas flow can not only be controlled, but they are also regulated. For this purpose, these devices are provided with a measuring device having sensors for the actual gas pressure 36 and the actual gas flow 38, and with a control unit 40 to which the output signals from these sensors are applied and which controls the pressure regulator and the flow regulator 34, as shown in FIG. 3.
However, a problem with the known devices for insufflating gas into a corporeal cavity is measuring the gas pressure, because it is usually impossible or only with great effort to measure the gas pressure directly inside the corporeal cavity. Due to the streaming gas and the drop in pressure (i.e. generated thereby), measuring the pressure outside the corporeal cavity leads to a measuring error, which cannot be readily tolerated in medical applications.
Therefore, it is proposed in that order to measure the "static" gas pressure P, to reduce the gas flow Q to the value 0 and to measure the gas pressure when the gas flow actually reaches the value 0 "statically". With regard to this, reference is made to FIG. 6 of U.S. Pat. No. 4,874,362 and the respective specification. Although this method has the advantage that it permits relatively accurate measurement of the gas pressure, it has the disadvantage that it does not permit an even relatively constant gas supply.
Furthermore, it has been proposed to calculate the pressure actually prevailing in the corporeal cavity from the output signal of the sensor while taking into account the flow resistance of the tube instrument and the insufflation instrument, such as a Verres needle, between the pressure sensor and the corporeal cavity whose pressure is to be determined.
However, in practice determining the flow resistance and the drop in pressure between the pressure sensor and the corporeal cavity has proven difficult.